The long-term goal of this project is to determine how the neurochemical circuitry of the retina contributes to its function of processing visual information. Attention will be directed to the identification of neuronal subpopulations which utilize specific transmitters or combinations of transmitters; elucidation of features which are unique to specific cell types, and characterization of synaptic input to identified cells. Specific aims include: (1) Use of immunocytochemical techniques to characterize the transmitter content and synaptic relationships of subpopulations of neurons in the cat retina. Antisera against neurotransmitters (glycine, GABA, glutamate, substance P and other neuropeptides) or their synthetic enzymes (choline acetyltransferase, tyrosine hydroxylase) will be used to localize the neurons which use these transmitters. Colocalizations of transmitters with other transmitters, modulators and marker enzymes will used to distinguish among multiple cell types which label for a single transmitter. Studies will be conducted at both light and electron microscopic levels in order to determine the laminar distribution nd synaptic relationships of labeled neurons. Receptor antisera will be used to explore the distribution of labeled neurons. Receptor antisera will be used to explore the relationships between presynaptic transmitter content and postsynaptic receptor distribution. (2) Diversity in synaptic vesicle morphology and protein content will be used to distinguish among multiple neuronal subpopulations utilizing a single neurotransmitter. (3) The synaptic input to identified neurons will be characterized through the use of intracellular filling in combination with postembedding immunocytochemical identification of transmitter input. Serial reconstructions will be used to relate this input to the dendritic branching patterns of receptive cells. (4) The neurotransmitter content of ganglion cells will be examined both within the retina and at projection sites in the superior colliculus and lateral geniculate nucleus. Ganglion cell identification will be assured by retrograde or anterograde labeling techniques. (5) Quantitative techniques will be used to estimate the relative amounts of transmitter in various retinal subpopulations and the effects of varying stimulus conditions on these levels. The elucidation of neurochemical and synaptic relationships as proposed in this application will contribute not only to an understanding of retinal function but will serve as a model for neurochemical interactions throughout the central nervous system.